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BeginnerWritten8 min

Reading a pattern before you cut

Nearly every first panel comes out bigger than the drawing, and the reason is always the same: the lines on a pattern are not the edges of the glass. They are the middle of the seam that will join it. Ten minutes on this now saves a panel that will not fit its frame later.

Three drawings, not one

A stained glass pattern is usually three documents that happen to look alike, and confusing them is where the trouble starts.

  • The cartoon is the full-size design: colors, shading, notes to yourself. It is what the finished panel should look like.
  • The cut line is the same drawing reduced to the lines that will be lead or foil. This is the one you work from.
  • The pattern pieces are the cut line chopped up, one paper shape per piece of glass.

Nessie gives you all three from one drawing, and keeps them in step when you change your mind. On paper you have to redraw, which is why people avoid changing their mind on paper.

The line is the middle of the seam

A drawn line has no width. The thing it stands for (a lead came, or two thicknesses of foil and the solder over them) very much does. The glass either side has to give up half of that width each.

cut linehalf the heartglassglass
The cut line runs down the center of the seam. Each piece of glass stops half a seam short of it.

For copper foil the allowance is small enough that many people ignore it on a suncatcher and pay for it on a window. 7/32 inch foil wrapped around 1/8 inch glass leaves roughly 1/64 inch (0.4 mm) standing proud each side. Over thirty pieces that is most of half an inch.

For lead came it is not ignorable. The heart of the came (the upright in the middle of the H) is typically 1/16 inch (1.6 mm). Every piece must be cut 1/32 inch (0.8 mm) short of the line on every edge that meets came.

Mark the pattern before you cut it up

  1. 1Number every pieceTop to bottom, left to right, on the cut line and on each pattern piece. Once forty paper shapes are loose on the bench, the numbers are the only thing telling you where they came from.
  2. 2Draw the grain arrowsOn any streaky, wispy or rippled glass, an arrow on the pattern piece says which way the movement in the sheet should run. Getting this wrong is invisible until the panel is up and the streaks disagree with each other.
  3. 3Mark the faceTextured glass has a right side. Decide now which face is out, and write it on the pattern. Flipping a piece late means it no longer fits its neighbors: it is the mirror image of the shape you wanted.
  4. 4Keep one clean copyYou are about to cut the working copy into forty pieces. Keep the whole cut line intact somewhere, because you will need to lay the glass back over it to check your fit.

Sanity checks worth thirty seconds

  • Does any piece have an inside curve tighter than your grinder bit? Decide now whether to cut it in two.
  • Is any piece a long thin spike? Anything under about 1/4 inch (6 mm) at its narrowest will break while you foil it.
  • Does every line either end at another line or at the border? A line that stops in the middle of a piece cuts nothing and will confuse you later.
  • Is the outside edge the finished size, or the glass size? Write down which, in the margin, in words.

The last one is worth being pedantic about. “Twelve by sixteen” means one thing to the person who has to fit a frame and another to the person cutting the glass, and they differ by the width of the border came.

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